let arr = [1, 2, 3];
let arr2 = [...arr]; // [1, 2, 3]
arr2.push(4);
console.log(arr2); // [1, 2, 3, 4]


// 数组含空位
let arr3 = [1, , 3],
    arr4 = [...arr3];
console.log(arr4); // [1, undefined, 3]
